osmo-gbproxy: Free msgb in ns2 prim callback

Commit cce88282388f in libosmocore changed the msg ownership model - the
callback that the user registers is now responsible for freeing the msg.

Change-Id: Iee940aba7d94afefb5957dbe5f0b04dcf951b31c
Related: SYS#4998
2 files changed